The present invention relates to novel processes for the preparation of 3,3-dimethyl-4-pentenoic acid (hereinafter "DPA") represented by the formula (I). ##STR1##
DPA of the formula (I) has been used for the preparation of permethric acid, an important intermediate for the preparation of pyrethroid insecticides. Various methods for preparing DPA are disclosed in JP patent laid open No. 77-42,853, No. 77-27,711, No. 76-131,819, No. 77-05,705 and No. 76-65,713. According to said inventions, DPA is prepared by hydrolysis of ethyl-3,3-dimethylpentenoate (hereinafter "EDP") which is synthesized by ether formation and Claisen rearrangement using 3-methyl-2-butene-1-ol (hereinafter "prenol") and triethylorthoacetate (hereinafter "TOA") in the presence of an acid catalyst, but said method is indicated as being undesirable because the prenol used therein can be prepared through only a very complicated procedure.
Some known methods for preparing prenol include for example, the first method German patent laid open No. 3021414, JP patent laid open No. 77-10207, and EP laid open No. 30109 disclosed a method for preparing prenol, in which isobutylene is reacted with formaldehyde, the obtained mixture is dehydrated to prepare isoprene, the isoprene is reacted with hydrochloric acid to form prenylchloride, the chlorine thereof is substituted for an acetyl group, and the resultant compound is converted into a prenol by deacetylation. The reaction formula of the above-mentioned method is as follows; ##STR2##
As a second method for preparing prenol, German patent laid open No. 2751766 disclosed another method, in which isoprenol is prepared as a mixture comprising 85% isoprenol and 5.9% prenol which is obtained by a reaction of isobutylene and formaldehyde under high temperature and pressure (Ref.: JP patent laid open No. 76-70,708), the resultant isoprenol is converted into prenol in the presence of a palladium catalyst to obtain a mixture of isoprenol and prenol having a ratio of 4:6, the mixture is fractionally distilled to separate prenol from it, and the residual isoprenol is converted into prenol by repeating the above process.
The steps of the above-mentioned process are represented as follows: ##STR3##
To obtain prenol, a starting material for the preparation of DPA, therefore, a multi-step process such as one of the foregoing must be carried out, and then there are many disadvantages in the procedure. Hereupon the present inventors expect that there will be many advantages in the process for preparing DPA, if DPA can be directly prepared from an isoprenol or a mixture of isoprenol and prenol.